I've been getting this about 30% of the time, and really don't have an idea why. Ultimately, the encode works if I just Export directly from Premiere. In AME I have tried cleaning the Cache and relaunching the system. Sometimes that works, sometimes not.
I'm running CC2015 on a late 2013 iMac max RAM & VRAM, RAID & latest Nvidia driver. Usually, timeline has ProRez, AVCHD &/or XAVC L, audio sweetened in Audition, sometimes MB Looks, others just Lumetri.
I've seen this pop up on blogs for years now, but never a solution.